The detection of optical radiation at infrared wavelengths has traditionally been difficult, even with state of the art optical detectors. Accordingly, it has typically been necessary to incorporate some form of amplifier system for amplifying the input infrared signal by a sufficiently high factor such that the signal can be more easily detected by existing optical detectors.
Present day amplifier systems used to amplify input infrared signals typically make use of a non-linear crystal. The non-linear crystal enables the physical process known generally as “parametric downconversion” to occur. In this process, a laser beam at a frequency ωp propagates through the non-linear optical crystal. Via a non-linear optical interaction, the beam at frequency ωp generates two other beams, the signal beam ωs and the idler beam ωi. These two beams are generated subject to the conservation of energy constraint wherein ωp equals ωs plus ωi. Immediately, it will be appreciated that the beam ωs and the beam ωi will be of lower frequency (i.e., have a longer wavelength) than the first beam ωp, which may also be referred to as the “pump” beam.
It is also possible to generate the above-described amplification process by co-propagating a weak input signal at frequency ωs along with a second beam at frequency ωp as they enter the crystal together. Using the same non-linear optical process, optical energy at frequency ωp will be transferred to the beam at ωs. The result will be that the strength of the signal ωs increases significantly after it reaches the opposite end (i.e., output end) of the non-linear optical crystal. One can therefore treat the combination of the crystal and the pump beam ωp as an amplifier for the optical signal ωs.
Even with the co-propagation of a weak input signal ωs along with a pump beam ωp, in most instances the interaction that occurs within the non-linear crystal is not sufficiently strong to provide the needed degree of amplification. Traditionally, this limitation has been overcome by operating the non-linear optical amplifier in a pulsed mode. Although the average optical power remains low, the peak power can be made quite high, on the order of several megawatts per square centimeter, so that the conversion efficiency occurring within the non-linear crystal during the duration of the laser pulse is quite good. This works well for amplifying the signal ωs if one has direct knowledge of its arrival time. For example, in active illumination cases, the signal pulse at ωs that is coming from the sample can be gated in time with the illumination source. However, if one wishes to amplify a continuous wave (CW) signal at frequency ωs the duty cycle associated with the pulsed laser format will lead to a loss of information over a large fraction of the ωs signal.
Therefore, ideally speaking, the amplifier should also be pumped in a continuous wave mode, thus allowing amplification of the ωs signals that arrive at random intervals. Since the continuous wave pump intensity, however, will be much lower than the pump intensity provided in the pulsed implementation, other means are required to improve the efficiency of a continuous wave amplifier system.
Therefore, there is a need for a continuous wave, non-linear optical amplifier which provides the needed degree of amplification to a continuous wave non-linear optical signal to enable detection of the amplified signal produced therefrom by a conventional optical detector.